How do I know what size cup to use for the rice test?

Although useful as rough guidelines and as communication tools, all the modalities currently used to predict what breast implant size/profile will best achieve a patient's goals with breast augmentation are not very accurate, in my opinion. The use of the rice test, sizers placed beneath a patient's bra, goal pictures, computer imaging technology… are all useful, but not necessarily as accurate as one would hope.
In my opinion, nothing will replace careful verbal preoperative communication with your plastic surgeon, preferably in front of a full-length mirror along with the use of as many "communication" tools” listed above.
In my practice, I use all of the above modalities and then use intraoperative temporary sizers to help determine the best breast implant size/profile to achieve a patient's specific goals as closely as possible. For this reason, I think it is helpful to have the entire range of breast implant  sizes/profiles available in the upper room.
Achieving realistic expectations prior to proceeding to the operating room is also an important part of the preoperative communication process. Patients should be aware that the results of their breast augmentation will not necessarily match exactly what they are visualizing with anyone of the above-mentioned communication tools.
Given your questions and concerns, and given that your surgery is coming up soon, I would suggest that you schedule additional time to spend with your plastic surgeon. This additional time spent will be helpful in the communication process and in alleviating some of your anxiety.
Best wishes; hopefully you will be very pleased with the outcome of the planned procedure.

The rice test involves adding different amounts of rice to either a baggy or a segment of panty hose to create basically a bean bag that you can add to your bra so you can see how you look. About 2.5 cups of rice should give you about a 600cc implant look. However, I think trying on the implants with your surgeon is the preferred method. I would not recommend going into your surgery confused about your size.